1. Field of the Invention
This invention relates to a method of controlling the generation of clinker ash from exhaust gas dust in a boiler, furnace or the like which employs dust coal as a fuel.
2. Description of the Prior Art
The use of boilers, furnaces, and the like which employ coal as a fuel has been increasing in recent years. However, because coal contains a small amount of volatile matter (20 to 30%) and an extremely high amount of fixed carbon (40 to 60%) as compared with heavy oil, it is less combustible. Therefore, recent types of coal-fired boilers and furnaces are designed to allow coal to be pulverized to less than 200 mesh (about 95%) in order to increase its activity and contact area with oxygen, thereby resulting in improved combustibility. Coal of low combustibility is fired in a blend with coal of greater combustibility.
Because coal has a much higher ash content (10 to 30%) than that of heavy oil, a great amount of ash is generated. For example, about 60,000 tons of ash per year is produced in a coal fired boiler of 500 T/H class. Coal ash is classified broadly into fly ash and clinker ash. Clinker ash is the ash which accumulates at a boiler bottom and comprises about 15% of the total ash quantity. The remainder is fly ash, which is collected in an air heater hopper and an electrostatic precipitator hopper. This ash contains mainly SiO.sub.2 and Al.sub.2 O.sub.3, with 15 to 20% or less of unburnt matter. The amount of ash produced may be roughly calculated from the ash content of a coal, but the properties of the ash generated vary with the type of coal.
A coal containing a large quantity of iron sulfide, because of its low melting point and high specific gravity, cannot be carried on a stream of gas and collides against furnace heating surfaces, resulting in accumulation of molten ash. This is referred to as slugging.
In the case of a coal containing a large quantity of alkali metal, because the compounds of alkali metal such as Na, K, etc. contained in the coal are activated due to reduction by carbon, carbon monoxide or hydrogen to react with SiO.sub.2 present on a heating surface and thus to produce low-melting alkali silicates (for example, Na.sub.2 SiO.sub.3), ash in the coal adheres to the resulting tacky water-glass-like substance, resulting in enlargement of the ash. This is referred to as fouling.
A combination of these conditions causes both slugging and fouling and results in enlargement of the ash in the form of a large clinder-like agglomerate formed on the burner throat or heating surface. FIG. 1 shows the positions in which these adverse effects take place. When these effects occur, the following problems result:
(1) the gas temperature of a furnace outlet may increase due to reduced absorption of heat by the furnace; PA1 (2) enlargement of the ash due to adhesion of a molten slug to a burner throat may cause blockage, resulting in burning problems in extreme cases; PA1 (3) a large agglomerate which has accumulated in a furnace may drop in the form of clinker, resulting in damage being caused to a waterwall tube; PA1 (4) rise in temperature of an exhaust gas as mentioned in (1) above and rise in temperature of the metals of which a superheater and a reheater are made may lead to an increased amount vapor spray, resulting in reduced boiler efficiency; PA1 (5) temperature difference between waterwall tubes increases due to adhesion of slug; and PA1 (6) an increase in the generation of clinker ash leads to its reduced flowability which in turn causes blockage and results in certain problems with respect to disposal of the furnace bottom ash. PA1 (1) blowing ash off with vapor from a sootblower; PA1 (2) reducing the temperature of a furnace, or reducing the temperature of a tube wall; PA1 (3) reducing the load; PA1 (4) changing the type of coal employed; and PA1 (5) absorbing any molten alkali metal present on a heating surface by means of an alkali metal-collecting additive, such as SiO.sub.2 powder or organic SiO.sub.2.
To prevent the occurrence of these problems, measures are taken to reduce loads or to change the type of coal employed. In an extreme case, operations may be stopped, and cleaning of the inside of a furnace, disposal of the clinker, and cleaning of the pipes has to be undertaken. Such a situation leads to serious financial loss.
In general, the ash which has dropped and accumulated at the bottom of a furnace is referred to as clinker ash but in the present specification this term also includes slug (ash) which has adhered to boiler heating surfaces.
Methods of removing molten clinker ash include the following:
However, these methods have the following disadvantages.
Whilst the above means (1) is effective, it is difficult to install sootblowers over the entire inside area of a furnace both from the physical and the economical viewpoints. Furthermore, because molten clinker is an adherent, it is difficult to remove slugging even when utilizing increased pressure. Excessive pressure or applying pressure with excessive frequency may cause a heating surface to suffer erosion and to be reduced in wall thickness, resulting in the bursting of a heating surface.
The above means (2) and (3) undesirably involve the reconstruction of a boiler or a reduction in efficiency.
The above means (4) has a certain advantage in that selection of a type of coal which is, for example, ##EQU1## provides reduced slugging, but is not entirely satisfactory.
In the case of the above means (5), powder injection of high-melting SiO.sub.2 and Al.sub.2 O.sub.3 inversely promotes slugging, bringing about an elevated exhaust gas temperature and increased melting. Although addition of SiO.sub.2 type additives has been performed with the aim of physically adsorbing any alkali metal substance on a heating surface, this measure does not represent a fundamental solution. Furthermore, these additives have no effect on FeS.sub.2 slugging. At present there are no effective additives available which aim at controlling clinker. Although iron oxide powder has in the past been charged directly from a burner into a boiler or a furnace for the purpose of reducing soot and dust, adhesion of particles to the heating surface in such cases was found to inversely promote slugging, resulting in an increased exhaust gas temperature. Thus, there has to date been no effective means for controlling the clinker ash produced by the burning of dust coal.